Particle board is typically made by depositing on a belt a thick mat of glue-coated particles, in the case of oriented strand board fibers between 75 mm and 150 mm long, preferably between 100 mm and 120 mm, and about 0.75 mm thick. The mat is then heated and pressed to the desired finished thickness. The pressing can be done continuously in an apparatus such as described in commonly owned U.S. Pat. No. 5,336,077 or in an intermittently operating platen-type press.
In order to minimize pressing time and, therefore, increase production efficiency while increasing the quality of the finished product, it is standard to preheat the mat before finish pressing it. German patent 1,276,912 describes how the mat is cut into sections and steam is used to heat the mat sections to the pressing temperature. To this end superheated steam at about 105.degree. C. is used. The mat sections are loaded into the opened platen press which is subsequently closed. Then the sections are at first pressed without the application of steam to a smaller thickness than the desired finished thickness. Subsequently the platens of the press are heated by passing the superheated steam through a steam chamber connected to at least one of the platens which is formed with holes or pores so that the steam can get through to the mat section being pressed. Such a system produces a finished product that is frequently of less than the desired high quality.
Accordingly German patent document 3,640,682 describes how the particle mat is produced continuously and is prepressed somewhat by means of an unheated double-belt press before being preheated. The prepressed mats are then supported on a foraminous belt arranged above and below chambers into which a heated fluid, preferably superheated or saturated steam, is fed under pressure so that it soaks into and heats the mats. The heat transfer with this system is poor so that treatment time is fairly long.
In general it is hard to tailor the preheating with respect to the desired temperature and moisture content of the particles, different adhesives, and different parameters of the finished product. The main reason for this difficulty is that the fluid used for the preheating frequently has undesired side effects, that it is can change critical parameters of the particle mat in an undesired manner.